THE LATERAL-LINE CAN MEDIATE RHEOTAXIS IN FISH

Rheotaxis is a behavioural orientation to water currents(1). It has be en demonstrated physiologically that some lateral-line receptors are p articularly well suited to provide information on water currents(2), b ut their contribution to rheotaxis has been largely overlooked. The ac cepted view is that rheotaxis is mediated by visual and tactile cues', and that in rheotactic orientation ''the lateral lines play only a mi nor role''(3). Here we provide a direct demonstration that rheotaxis c an be mediated by the lateral line, and indeed by one specific recepto r class of this system. In three diverse fish species, pharmacological block of the entire lateral-line system substantially increases the v elocity threshold for rheotactic behaviour. The same effect is observe d when only superficial neuromasts are ablated, whereas blockade of th e other receptor class, canal neuromasts, has no such effect. Our resu lts therefore demonstrate that superficial neuromasts make an importan t contribution to rheotactic behaviour in fish.